Hearing instruments generally offer both an omnidirectional and directional mode of operation. The omnidirectional mode is executed with a single omnidirectional microphone. The directional mode is often executed with a single, passive, differential microphone having both a front and rear acoustical conduit. The rear conduit may contain an acoustical resistance in the form of a screen or mesh that is engineered to provide a fixed sensitivity pattern such as a cardioid, hypercardioid, etc. Two separate microphones are thus used to provide the two modes of operation in a hearing instrument. There exists, therefore, a need for a system to provide both modes of operation in a smaller profile, at lower cost, with the option of adjusting the acoustical resistance by adjusting the orifice dimensions electromechanically. There also exists a broad class of materials referred to as electroactive, conductive, or conjugated polymers that can be electrically controlled to produce large linear, volumetric, or bending strains when configured as an actuator under a DC voltage. These electroactive polymers (EAP) can be configured to operate as an acoustical valve in a small, low-cost, omni and directional microphone system.